Mediating roles of sleep quality and resilience in the relationships between chronotypes and mental health symptoms

Sleep and mental health are intrinsically intertwined, but not every individual with problems sleeping develops a mental health disorder. This study examined the association among chronotypes, resilience, sleep quality and mental health symptoms amongst otherwise healthy individuals. Two hundred adults (Mage = 27.75 ± 5.11, 68% female) with no previous diagnosis of mental illness were recruited and filled in a set of questionnaires measuring chronotypes, sleep quality, depression and anxiety symptoms. The findings from the path analysis showed that the morning type had a statistically significant direct effect on a range of sleep quality indices. These included better subjective sleep quality, shortened sleep latency, and fewer daytime dysfunctions, as well as a higher level of resilience. However, it did not significantly affect depression and anxiety symptoms. In addition, the morning type had statistically significant indirect effects on a higher level of resilience and fewer depression and anxiety symptoms through the mediating effect of sleep quality indices. Findings from this study support that morning type is associated with better resilience and psychological health, which is mediated through better sleep quality.


Chronotypes and sleep quality
Chronotype has strong implications for sleep quality, with evening type reporting a range of sleep complaints, such as more frequent nightmares, shorter sleep duration, more frequent use of sleep medication, poorer sleep quality, and longer sleep onset latency than morning or intermediate types [25][26][27][28] .It is increasingly recognized that adequate sleep is required for mental health, and poor sleep quality is a risk factor in the development and maintenance of mood disorders 29,30 .Problematic sleep has also been shown to exacerbate symptom severity 31 .Nevertheless, studies on the impact of chronotype on mental outcomes and its relation to sleep quality are both limited and controversial.Previous longitudinal studies have reported that poor sleep quality was a predictor of depression 17 .In a healthy sample, Horne et al. 32 observed a significant link between evening type and increased depressive symptoms, and this relationship was partially mediated by sleep quality.Levandovski et al. 33 observed a relationship between sleep quality and depressive symptoms in addition to the eveningness-depression association.These findings provide preliminary evidence that mental health symptoms might be associated with chronotype, and sleep quality might mediate the relationship between chronotype and these symptoms.
However, several studies have suggested that chronotype might be a separate risk factor for mental health symptoms, beyond other sleep-related problems.For example, some studies have shown that sleep quality and sleep complaints did not mediate the relationship between evening type and depressive symptoms in healthy subjects 34,35 .Müller et al. 36 reported that evening type and poorer subjective sleep quality were separately associated with higher depression severity in depressed patients.These studies might suggest that sleep quality alone is insufficient to fully explain the relationship between chronotype and depression, and additional factors are expected to play a role in better understanding the link between chronotype and mental health symptoms.Psychological resilience (hereafter called resilience) could be one of the important inter-players between chronotypes and depression.

Resilience
Resilience is defined as the ability to adapt to change and the capacity to face disturbance 37 .It is a variable that consistently appears in the literature in relation to chronotype and mental health.Evidence shows that resilience works as a protective factor for a number of mental illnesses, including depression and anxiety 38 .Low levels of resilience are associated with a higher likelihood of developing a range of mental health problems 39,40 .Chronotypes are associated with resilience 41,42 .For instance, participants with morning type showed the highest resilience level, followed by the intermediate and evening types in a large sample of 1,922 student and adult participants 41 .Findings from another study of 1,094 Korean college students suggested that morning type was a significant positive predictor of resilience, even after controlling for age, sex, depressive and anxiety symptoms, and sleep quality 42 .Similar findings were reported in patients with major depressive disorder (MDD) after controlling for the influence of age, gender, length of education, economic status, onset age, and suicide attempt history 43 .

Aim of the study
In summary, research studying the relationship between chronotype and mental health symptoms commonly investigates a singular relationship.However, the relationship between these constructs is complicated.Thus, it is crucial to explore the underlying factors that mediate the association in order to guide the development of future interventions to ease the strain posed to the health care system.The aim of this study was to understand the role of sleep quality and resilience in mediating the relationships between chronotype and mental health symptoms in a sample of healthy Chinese adults with no previous diagnosis of mental illness.

Results
The demographic details are listed in Table 1.According to Buysse and colleagues 44 , subjects with a global PSQI score larger than five indicate an association with poor sleep quality.The mean global PSQI score of our participants was 6.26 ± 2.88, indicating that our participants had certain sleep problems.Twenty-seven (13.5%) participants were regarded as morning type, 130 (65%) participants were intermediate type, and 43 (21.5%)participants were evening type.The mean scores of DASS for depression and anxiety symptoms were 5.04 ± 4.65 and 5.66 ± 3.94, respectively, suggesting mild level of depression and moderate level of anxiety symptoms in our participants 45 .The Spearman's rho correlation coefficient is shown in Table 2.

Path analysis
Chi-square statistics as well as the multiple mode fit indices indicated that the unconstraint model fitted our data.Chi-square difference tests further suggested that both the MEQ constraint model (Δdf = 6, Δχ 2 = 12.397, p = 0.054) and the PSQI constraint model (Δdf = 6, Δχ 2 = 4.232, p = 0.645) were significantly better than the unconstraint model.However, in the MEQ constraint model, some of the fit indices did not meet the cut-off criteria (i.e., TLI < 0.95, RMESA > 0.05, Table 3).We therefore considered this MEQ constraint model not fitting our data well.The proceeding analyses supported that the PSQI constraint model had the best fit to our data.Therefore, path analysis was conducted in this PSQI constraint model.The final model is illustrated in Fig. 1.  observed, whereas the direct effects of the MEQ scores on the level of depression and anxiety symptoms were non-significant.Since the effect of the PSQI subscales on CD-RISC was constraint to be equal, the unstandardized β across the seven subscales of PSQI on CD-RISC was the same, which was found to significantly predict CD-RISC (Unstandardized β = − 1.064, Bootstrap SE = 0.280, 95% CI = − 1.622 to − 0.522, p < 0.001).

Discussion
Sleep struggles and mental health difficulties are intrinsically intertwined and involve multiple pathways.Adding to literature that has predominantly focused on insomnia and disorders, this study examined chronotypes and mental symptoms amongst otherwise healthy individuals, as well as the possible mechanisms of resilience and sleep quality.
The current study has first explored the behavioral differences across chronotypes.Similar to the distribution/ pattern reported by Zhang et al. 8 , the majority of our participants reported being of the intermediate type (65%), www.nature.com/scientificreports/followed by the evening type (21.5%), and morning type (13.5%).Comparing resilience level, sleep quality, and psychological symptoms, the morning type demonstrated statistically significant higher resilience levels than the other two chronotypes; no other differences were however identified.The effect size of such difference was considered medium based on Cohen's f statistics.Moreover, it is surprising to note that, although our participants had no previous record of mental illness, they showed generally elevated psychological symptoms and poor sleep quality compared to international norms 46 .Specifically, on average, they demonstrated mild levels of depression and moderate levels of anxiety symptoms, echoing a recent study using a telephone survey in Hong Kong, which reported that highly stressed working environments and long working hours are conducive to poor sleep quality and reduced mental well-being 47 .Prevalence estimates of sleep quality in typical Western populations are well documented, but data for Chinese adults are comparatively scarce 48 .The current study provides additional evidence to support the high level and prevalence of negative psychological symptoms and poor sleep quality.
To understand the relationships between chronotypes and mental symptoms, our study suggested a new perspective and identified a complete mediation model between the two constructs.In other words, chronotypes have no direct effect on either level of depression or anxiety symptoms when controlling for the impact of sleep www.nature.com/scientificreports/quality and resilience.Specifically, path analysis suggested that the relationships among these domains are unique and complex.In exploring the mediating role of sleep quality, the direct effects of chronotype are observed in some of the indices of sleep quality, such as subjective sleep quality, sleep latency, use of sleep medication, and daytime dysfunction.However, the effect size of these direct effects was considered small (unstandardized β < 0.1 in magnitude), the practical significance of the effect of chronotype on sleep quality remains questionable.These findings suggest that the propensity for an individual to sleep at a particular time may have minimal influence on various sleep parameters including sleep quality.It remains unclear whether circadian disruption is more severe in the evening type resulting in more sleep disturbance.This requires future studies to further confirm.In addition, caution must be taken for the interpretation of our results in the use of sleep medication.It is because there were only six participants rated 1 in this aspect, while the rest of the 194 participants indicated that they did not use sleep medication at all.Among the six participants who rated 1 in that area, five of them were in the intermediate-type and one of them was in the morning-type.These findings corroborate previous research that participants of the evening type were reported to have longer sleep latency, poorer sleep quality, and greater daytime dysfunction than participants of the morning or immediate types 49 .This might be due to individual differences stemming from their lifestyle preferences; there is a tendency for evening types to have irregular sleep habits or erratic sleep-wake schedules.Evening types were correlated with later bedtime and wake-up time on weekends, shorter time in bed during the weeks, and longer weekend time in bed 50 .It has also been reported that participants with evening types tend to consume more alcohol, caffeine, and tobacco, resulting in more frequent daytime sleepiness than participants with morning types 51,52 .
Direct effects of sleep quality on depression and anxiety symptoms are also observed, using parameters such as sleep duration, habitual sleep efficiency, sleep disturbance, use of sleep medication, and daytime dysfunction, all of which are associated with anxiety symptoms; however, only sleep duration and daytime dysfunction are significant in predicting levels of depression.It is suggested that these parameters related to subjective sleep difficulties may have unique implications for the mechanism delineating between chronotype and mental health symptoms.Our findings cohere with previous research that daytime dysfunction and shorter sleep duration were positively associated with depression 53 .It is possible that daytime dysfunction could stem from insomnia, a major symptom of depression 54 .Individuals sleeping for a short period of time may lead to insufficient rest and a greater perceived stress severity, which has been reported as a risk factor for depression 55,56 .In fact, an indicator of a healthy lifestyle could be good sleep conditions and mental well-being.
Our findings also discovered the mediating role of resilience in explaining the relationship between chronotype and mental health.While the role of resilience remains unclear, other recent studies found that sleep disturbance 57 , sleep quality 58,59 , social support 60,61 , and rumination 62 were significant mediators of the relationships between chronotype and depression and anxiety symptoms.Nevertheless, our study found that participants with morning type had higher resilience than participants with evening type, confirming previous research showing that morning types demonstrate stronger resilience than evening types among healthy adults 41 .
It appears that chronotype affects resilience; this may be due to disrupted daily routines 63,64 and circadian rhythms 65,66 .Strong resilience requires adaptability to work in social situations as well as psychological and physical conditions 37 .It is clear that individuals with high resilience are able to cope with challenging situations, and resilience is demonstrated to be protective against depression in college students 67,68 .A Korean study 42 found that morning chronotype and longer daytime exposure to sunlight between 10:00 and 15:00 were significantly associated with greater resilience.In contrast, individuals of evening chronotype are more likely to experience social jetlag caused by desynchronized biological and social clocks, resulting in lower resilience 41,69 .In turn, sleep problems and reduced resilience are more likely to facilitate the development of mental health issues.This is in accordance with a meta-analytical review that resilience was negatively correlated with indicators of ill-being (e.g., anxiety, depression, negative affect) and positively correlated with indicators of well-being (e.g., life satisfaction, positive affect) 70 .
Another possible explanation is that the contribution of family, friends, and colleagues to social support may help explain the relationship between evening chronotype and depression 61 .It has been shown that morning chronotypes have a higher level of social support than evening chronotypes.Individuals who belong to the evening chronotype have conflicts in their social lives, with feelings of loneliness and low support from others 60 .While evening chronotypes may have adequate social support, they may not be aware or willing to acknowledge it due to negative cognitive bias 60 .However, prior to concluding these findings, future research is now required to determine whether social support helps explain the relationship between chronotypes and mental health in Chinese healthy adults.
Statistically, we demonstrated significant direct paths linking the three constructs, whereas the effect size of chronotype on subjective sleep quality was rather small.Similarly, sleep quality had statistically significant mediation effect on the relationship between chronotype and resilience, although the effect was relatively small.The magnitude of direct and indirect effects should be re-evaluated in a bigger sample with an equal number of participants in each chronotype.Also, circadian disruption should be considered as an additional parameter in future mediation models.Nevertheless, sleep is an important brain function for memory consolidation, brain reactivity, and emotional regulation 71,72 .A 60% increase in amygdala activity in response to emotional stimuli was identified under conditions of poor sleep quality, which leads to reduced resilience 73 .Our findings are novel, as no previous studies have examined the role of sleep quality in explaining the relationship between chronotype and resilience.It is crucial to reaffirm the importance of sleep quality to improve resilience and mental health, as previous studies found that psychological well-being problems were less likely to occur with normal sleep quality 74 .People with better sleep quality also had higher resilience 75 .

Limitations and conclusions
The current study has some limitations that should be taken into consideration and points to suggestions for further research.This study employed a cross-sectional approach which could not make causal inferences.A longitudinal approach is needed to verify the mediation model presented in this study.The measurements of sleep quality, depression and anxiety symptoms were self-reported using questionnaires.It is unknown whether the participants actually had irregular patterns.A recent study 76 revealed a U-shaped association between sleep duration and depression: Both insufficient sleep (< 8 h) and excessive sleep (> 8 h) increase the risk of depression.Researchers may wish to consider using more accurate wearable devices (e.g., actigraphy, mental health technologies such as smartphone applications) for an objective measure of sleep quality and using an observational approach to identify depression and anxiety symptoms 77 .It is also worth noting that the participants in this study had a higher education background with approximately 78% of them holding a bachelor's degree or above.Our findings may not be generalizable to those with a lower education background.Finally, this study only used age as the controlling variable-there might be other confounding variables that have not been controlled for due to data availability (e.g., medication usage, occupation, socioeconomic status).
Both sleep and mental health are critical aspects of daily life, highlighting the relevance of this complex relationship and the pressing need to unravel it further.This study not only shows that chronotype and sleep quality are associated with depression and anxiety symptoms, but also unearths the mechanistic roles of sleep quality and resilience in the relationship between chronotype and mental health.Our findings suggest that cultivating resilience while improving sleep hygiene with consideration of different chronotypes may be a viable intervention opportunity for improving well-being.

Participants
Two hundred adults (M age = 27.75 ± 5.11; 136 females and 64 males) participated in this study.The demographic details are listed in Table 1.Snowball sampling method was adopted.Participants were recruited via social media (e.g., WhatsApp).Inclusion criteria were mentally healthy adults without a history of psychiatric or psychological disorders; substance and/or alcohol abuse that might influence their sleeping quality and/or mental health status.Participants were required to declare in the online form if they had the abovementioned issues before joining the study.Data collection was completed in March 2022, and the data analysis was completed in January 2023.The study procedure was approved by the Research Ethics Committee of The Education University of Hong Kong (EdUHK; reference number: 2018-2019-0117-01).All experiments were performed in accordance with relevant guidelines and regulations.

Study design
All participants were well informed of the study aims and procedure through an online information sheet, they provided an online informed consent before participating in the study.Participants then filled in a set of online questionnaires including Morningness-Eveningness Questionnaire (MEQ) 78 , The Pittsburgh Sleep Quality Index (PSQI) 79 , Connor-Davidson Resilience Scale (CD-RISC) 80 , Depression Anxiety Stress Scales (DASS) 81 , and sociodemographic details.

The Pittsburgh Sleep Quality Index (PSQI)
The PSQI assesses seven dimensions of sleep quality over the past month: subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbance, use of sleep medication, and daytime dysfunction 79 .A global PSQI score larger than 5 is regarded as having sleep problems.Higher scores indicate more difficulties in sleep.

Connor-Davidson Resilience Scale (CD-RISC)
The CD-RISC measures the overall perceived resilience of oneself 80 .It has been reported as a reliable tool to assess resilience 82 .It consists of 25 items rated on a 5-point Likert scale from 0 (not true at all) to 4 (true nearly all the time).Higher scores represent higher resilience.

Depression, Anxiety and Stress Scale (DASS)
The DASS measures the affective states of depression, anxiety, stress, and general psychological distress 81 .It consists of 21 items with 7 items in each subscale.The depression (DASS-depression, 7 items) and anxiety (DASSanxiety, 7 items) subscales were used in this study.Participants were asked to rate from 0 (did not apply to me at all) to 3 (applied to me very much).Higher scores indicate more severe symptoms of that area in the past month.

Figure 1 .
Figure 1.The finalized Pittsburgh Sleep Quality Index (PSQI) constraint model.Each exogenous variable includes an error term (e1-10).The effect of seven subscales of the PSQI on Connor-Davidson Resilience Scale (CD-RISC) was constrained to be equal, which is represented by the dotted line with an arrowhead.These subscales are subjective sleep quality (Qua), sleep latency (Lat), sleep duration (Dur), habitual sleep efficiency (Eff), sleep disturbance (Dis), use of sleep medication (Med), and daytime dysfunction (Dys).Covariations between errors are illustrated by dotted lines with double arrowheads.MEQ, Morningness-Eveningness Questionnaire; DASS, Depression Anxiety Stress Scale.

Table 2 .
Spearman 's rho correlation among all variables.Numerical values represented Spearman's rho correlation coefficient.Bonferroni corrected p-value = 0.00048 (0.05/105 comparisons).Significant correlations were bold.MEQ, Morningness-Eveningness Questionnaire; CD-RISC, Connor-Davidson Resilience Scale; DASS, Depression Anxiety and Stress Scale.Higher MEQ scores indicate a higher tendency towards the morning type.Higher scores in each of the Pittsburgh Sleep Quality Index (PSQI) components indicate greater problems in that particular aspect.